Zodiacal Light Calculator: Documentation

Overview

This is a brief introduction to the Zodiacal light as implemented in the Zodiacal Light Calculator. For a more detailed description of the zodiacal light, please see the references cited below.

The zodiacal light is mainly due to sunlight scattered by interplanetary dust grains. Because the Sun is a cool star, there will not be much contribution from the zodiacal light in the UV, particularly in the FUV.

The zodiacal light will effectively be a smooth background over the image plane. The level of the zodiacal light will be dependent on the helioecliptic longitude and β, the angle from the ecliptic.

On-line Calculator

The on-line calculator is a front end to the C program. The only inputs required are the date and the observing direction. The output is the zodiacal light spectrum in units of photons cm-2 s-1 sr-1 Å-1 plotted as a function of wavelength. The spectrum itself can be downloaded in a number of formats from the link provided in the output section.

Implementation

Problem Statement

In order to calculate the zodiacal light, we need:

Sun position

Zodiacal light spectrum

Zodiacal distribution

Sun Position

Spatial Dependence

The spatial dependence of the zodiacal light has been tabulated by Leinert et al. and is reproduced here. The heliocentric longitude increases with row number and the β angle increases with column number. The units of the zodiacal light are 10-8 W m-2 sr-1 µm-1 at a wavelength of 500 nm. The scale factor to convert these units into photons cm-2 s-1 sr-1 Å-1 is 252 at 5000 Å; ie, the numbers in the table have to be multiplied by 252.

Zodiacal light as a function of ecliptic coordinates

β

0

5

10

15

20

25

30

45

60

75

90

0

3140

1610

985

640

275

150

100

77

5

2940

1540

945

625

271

150

100

77

10

4740

2470

1370

865

590

264

148

100

77

15

11500

6780

3440

1860

1110

755

525

251

146

100

77

20

6400

4480

2410

1410

910

635

454

237

141

99

77

25

3840

2830

1730

1100

749

545

410

223

136

97

77

30

2480

1870

1220

845

615

467

365

207

131

95

77

35

1650

1270

910

680

510

397

320

193

125

93

77

40

1180

940

700

530

416

338

282

179

120

92

77

45

910

730

555

442

356

292

250

166

116

90

77

60

505

442

352

292

243

209

183

134

104

86

77

75

338

317

269

227

196

172

151

116

93

82

77

90

259

251

225

193

166

147

132

104

86

79

77

105

212

210

197

170

150

133

119

96

82

77

77

120

188

186

177

154

138

125

113

90

77

74

77

135

179

178

166

147

134

122

110

90

77

73

77

150

179

178

165

148

137

127

116

96

79

72

77

165

196

192

179

165

151

141

131

104

82

72

77

180

230

212

195

178

163

148

134

105

83

72

77

Spectral Effects

The zodiacal light is reddened but by not more than 20% so a first approximation is to simply use the solar spectrum from Colina et al. This spectrum is shown in Fig. 1 with a normalization described below.

Fig.1 Zodiacal light from UV to IR

Although there are indications that the colour (the brightness relative to the Sun) of the zodiacal light is dependent on both wavelength and position, I have assumed that the colour is unity. The Colina et al. [spectrum] has been scaled such that the value at 5000 Å is 252, corresponding to 10-8 W m-2 sr-1 μm-1 m-1 at 5000 Å. Thus the spectrum simply has to be multiplied by the appropriate scale factor from the Table, with a colour correction if desired.

Input/Output

All inputs to the program need to be specified in the parameter file called zodiacal_initparams.txt. This is a plain text/ascii file, and internally documented well. If the parameter file does not exist, the program will generate one with default input values so that users can modify it as per their needs.

The output of the program is the zodiacal light at the specified coordinates and date in units of photons cm-2 s-1 sr-1 Å-1 and should be good to within 20%. More accurate values could be obtained by adding a colour to the spectrum.